This invention relates to solutions of fluorine-containing polyisocyanates useful for impregnating porous inorganic substrates, to polyisocyanates suitable for this purpose and to alcohols suitable for the production of these polyisocyanates.
Porous inorganic substrates, such as tiles, fireclays, building bricks obtained from natural rocks, ceramics, marble, concrete, plasters, etc., undergo gradual erosion which leads to porosity, loss of binder and loss of mechanical cohesion in the inorganic material. This erosion process can be greatly accelerated by harmful environmental influences such as, for example, vehicle emissions or traces of acid in rainwater. With buildings of particular historical significance or of special interest, it is essential to keep their original appearance intact because renewal or replacement is undesirable.
It has already been proposed (DE-OS 3 629 061) to use isocyanate (NCO) prepolymers containing polysiloxane units to impregnate porous inorganic substrates. Although the recommended impregnating agents effectively hydrophobicize the treated substrates, they are still unsatisfactory in regard to oleophobicization which is equally important. That is, the polysiloxane-containing NCO prepolymers do not afford entirely satisfactory protection against the influence of aggressive organic contaminants of the type which enter the environment, for example, through the inadequate combustion of heating oils or fuels.
Polyisocyanates containing perfluoroalkyl groups are also known. For example, DE-A 0 435 641 describes polyisocyanates containing urea groups and fluorine which are recommended for impregnating ceramics or concrete or textiles. However, urea groups introduced by partial reaction of the isocyanate groups with water, adversely affect penetration into the porous mineral and, result in a poorer long-term effect. In addition, the urea groups adversely affect the water-repelling properties of the impregnated substrate.
Japanese Patent Application 67-21 331 (C.A. 69 (1968), Abstract 11 065a) describes water-repellent fluorine-containing polyisocyanates based on linear partially fluorinated alcohols and an aromatic polyisocyanate. This application does not, however, suggest that the disclosed polyisocyanates be used for impregnating porous inorganic substrates.